My method is described as follows:
1. According the input data, for each element, I defined two integer which are the number of edge pointing to that node ( say it A ) and the number of edge where that node points( say it B).
If A = 0, then that node is defined as root. Therefore, if finding root again, this is not a tree. Of course, if no root, this is also not a tree.
If A >= 2, then this is not a tree.
There is another occasion. That is when there are two graph, one is a circuit and the other is a tree. Then the result is wrong following the rules above. Therefore, after checking the rules shown above, I trace each node from the root, and get the number of checked node. If the number of the checked node is not equal to the number of nodes, then the graph must be separated and this is not a tree. Otherwise, it does.
However, I still get wrong answer ... >< ..
I don't know what mistake I make or I ignore some facts.
Plz help me >< ...